I am a senior in high school. This semester(2nd), I dropped out of IB English 12 with a C. The UCs have asked for a reason and it needs to be good.
I was thinking of saying: A reason I have dropped out of IB English 12 is that I am taking the AP test for English and not IB. IB English 12 literature has little correlation with the literature of the AP exam. I am also not going for the Full IB Diploma. I wanted to drop out of IB English 12 to reduce the workload in order to be able to keep up with my extracurriculars(gym,guitar, helping my family) and my online course for Electrical Engineering(MIT open courseware). Also, I wanted to drop out because I want to improve my writing that IB English 12 does not focus on doing. I will be self-studying for the AP English Exam by reading AP literature, studying new vocabulary everyday, and improve my writing in general. I am not leaving because it was too rigorous for me, but rather because I could not keep up with the workload with all my other AP classes. Now, it is strange that I say that I am switching out of IB English 12 because of its irrelevance to the AP exam but also taking IB Physics SL. The reason for this is because AP Physics was full. I talked with my teacher and looked at the curriculum between the two on College Board. and both AP and IB Physics learn about the same things with a little distinction on some sections. My teacher told me that I just need to learn the AP Physics sections that IB Physics have not gone over. Thank you.
Is this good enough of a reason or I need to add other things because if it’s not good enough it will affect me negatively!
Honestly, if you need that much to explain it, it’s going to sound like an excuse. Also if the C is on your transcript already, UC’s won’t just overlook that as they have the largest applicant pool in the nation combined by far, and limited spots. It’s better to just take the C rather than making an already exasperated team of adcoms read another lengthy text, and see you negatively.
Your reasons aren’t that good, especially since there’s a lot of them it looks like you hobbled together a bunch of small factors. By saying that IB English 12 doesn’t improve writing, you put your opinion against that of the tried and true IB curriculum. You saying you are studying AP Lit is meaningless, because there’s no proof to your claims which is exacerbated by the fact that you have many other reasons diluting the strength of your argument. When you say you are leaving not because it was too rigorous, you directly say next you couldn’t keep up. This appears to be the true reason, rather than a hasty claim to AP Lit, and you attach mistakes in IB English 12 to other courses by saying that you couldn’t keep up with other courses because of it, inferring that you can’t handle the other courses as well. You provide little concrete reason as to why you want to drop this course, and it definitely shows, especially since you kept IB Physics SL.
I’m sorry, but if I were reading this as an adcom, I would doubt your tenacity as a student. Best to fess up to your mistakes, as I’m sure this was probably just a bump in the road for you and you found success elsewhere. 
I agree with djinforthewin. Sounds like a lot of excuses. Just be honest: You over extended and college applications probably pushed you over the top - hence the C. Its not likely that your admission will be withdrawn if you explain that it was one-off and you are taking a less demanding English class. Obviously, based on your info, you aren’t planning on being and English major or journalism student so this isn’t crucial to your future (and I assume your other grades were good/stable and didn’t reflect and overall downward trend/senior slum).
One thing that stuck out to me is you saying that you didn’t drop out because it is too rigorous, but because you couldn’t handle the course load. Seems like the same sort of excuse to me and I would try to think of something better, or just like others have said in this thread, cut a lot of the excuses.
I think the first answer is spot on.
No on cares about self study or take an online no-credit class–that should not be a priority over your core classes. I am very confused if you are in an English class at all. I think it is a problem for UC if you are not. I think they would rather see you in an English class than take an AP test.
This is a lot of confusing–I don’t even know why you are talking about physics–you lost me there. Streamline this.